1. Field of the Invention
This invention relates to a method for the preparation of insulating oil from a paraffin- or mixture-base crude oil.
More particularly, this invention is concerned with a method for the preparation of novel insulating oil having good oxidation stability, electric characteristics and resistance to copper corrosion which comprises subjecting a distillate within a temperature range from 250.degree. to 400.degree. C. in terms of the boiling point at atmospheric pressure to solvent refining to a desulfurization from 30 to 75% by weight to give a raffinate, said distillate having been obtained either by distillation at atmospheric pressure of a paraffin- or mixture-base crude oil or by distillation under reduced pressure of a residual oil from the atmospheric pressure distillation, subjecting said raffinate to hydrogenating refining to a desulfurization from 40 to 90% by weight and subjecting the refined product to solvent dewaxing, and if necessary, subsequently subjecting the dewaxed product to clay treatment to a sulfur content of the final product from 0.1 to 0.35% by weight.
2. Description of the Prior Art
A number of methods are known for the preparation of mineral insulating oil. Traditionally, insulating oils have been manufactured from naphthenic crudes. The prior art methods employing naphthene-base crude oil as the feedstock could not be used for the preparation of electric insulating oil with satisfactory properties from paraffin- or mixture-base crude oil.
Among these prior art methods there are mentioned those methods which involve sulfuric-acid treatment, solvent refining or hydrogenating refining followed by treatment with a solid adsorbent for removing impurities such as unsaturated hydrocarbons, asphalt materials, sulfur compounds and nitrogen compounds. With lower degrees of refining, these treatments would result in products with no improvement in resistance to copper plate corrosion and electric characteristics and higher degrees of refining are required. With a higher degree of refining, however, there will usually be induced too much removal of natural oxidation-inhibiting components originally present in mineral oil to produce insulating oil with good oxidation stability, the above-mentioned problems are improved. In this respect, addition of a specified proportion of a lowly refined oil to the highly refined oil has been proposed for the preparation of insulating oil with good electric characteristics and oxidation stability. In Japanese Patent Publication No. 2981/1960 is described a method for the preparation of electric insulating oil with high oxidation stability which comprises adding to the refined oil a polynuclear aromatic component separated by extraction from mineral oil. There is also described in Japanese Patent Publication No. 3589/1966 production of electric insulating oil with high oxidation stability by addition to the refined oil from hydrogenation of a raffinate containing aromatics in a proportion of 23% by weight or lower a lubricating oil with a higher content of aromatics in a proportion of 15% by weight or lower.
As described above, the insulating oil has heretofore been produced using good-quality naphthene-base crude oil as the feedstock. In recent years the continuing short supply of naphthenic crudes have created a need for production of insulating oil from paraffin- or mixture-base crude oil. On the other hand, application to paraffin- or mixture-base crude oil of the prior art methods for naphthene-base crude oil as they are does not produce insulating oil possessing good oxidation stability, electric characteristics and resistance to copper plate corrosion. The naphthene-base crude oil and paraffin- or mixture-base crude oil being quite different from each other in properties, this is unavoidable and novel methods must be developed for the preparation of insulating oil from paraffin- or mixture-base crude oil. It is to be noted, for example, that when a paraffin- or mixture-base crude oil is treated in the same way as described in Experiment A in Example 1 of Japanese Patent Publication No. 18584/1961 using hydrogenating refining and clay treatment there will not be produced electric insulating oil with high corrosion resistance and oxidation stability.